1. Field of the Invention
The present invention relates to an optical recording medium having a phase-change optical recording layer and capable of high-density recording such as rewritable DVD (Digital Versatile Disc), which may be referred to as “optical information recording medium” or “optical disc” and relates to an optical recording method and an optical recording apparatus using the optical recording medium.
The present invention also relates to a multi-layered optical recording medium having two or more information layers each containing a recording layer in which information can be rewritably recorded and reproduced by irradiating the information layers with a laser beam to induce optical changes onto a material for the recording layers and relates to an optical recording method and an optical recording apparatus using the multi-layered optical recording medium.
2. Description of the Related Art
Recently, demands for high-speed recording of optical recording media have increased. Particularly for disc type optical recording media, speeding-up of recording is progressing, because recording speed and reproducing speed thereof can be speeded up by accelerating the rotation speed. Among optical discs, optical recording media capable of recording information with only modulation degrees of light for irradiation during recording enable price-reduction of media themselves and recording apparatuses because of the simple recording mechanism thereof and ensure high-compatibility with reproducing-only apparatuses, because intensely modulated light is also used for reproducing. For the reason stated above, these optical recording media became popular, and demands for high-densification and high-speed recording are further increased.
Among these optical discs, those using phase-change materials have become mainly used because of the capability of repeatedly rewriting information many times. In the case of optical discs using phase-change materials, information is recorded by subjecting the recording layer material to a quenched condition and an annealed condition by means of intensified modulation degrees of light for irradiation. In a quenched condition, the recording layer material becomes amorphous, and in an annealed condition, the recording layer material becomes crystalline. Since optical properties differ between an amorphous phase and a crystalline phase, information can be recorded. In other words, phase-change optical recording media are those in which a thin recording layer on a substrate is irradiated with a laser beam to heat the recording layer and induce phase changes between a crystalline phase and an amorphous phase on the recording layer structure to change the reflectance of the disc to thereby record or erase information. Typically, non-recorded portions are formed in a crystalline phase having a high reflectance, and information is recorded by forming marks formed in an amorphous phase having a low reflectance and spaces formed in a crystalline phase having a high reflectance on the non-recorded portions.
Since a complex mechanism of “quenching” and “annealing” of such a recording layer material is used as a recording principle, in high-speed recording, as commonly known, information is recorded by irradiating a recording medium with a recording light beam of which the pulse is split and intensely modulated into ternary.
Examples of an emission pattern of a waveform or a recording strategy for repeatedly recording data which contains marks and spaces include those used for DVD+RW or the like as shown in FIG. 7. A mark in an amorphous condition is formed by pulse irradiation of repeatedly alternating a peak power (Pw=Pp) beam and a bias power (Pb) beam, and a space in a crystalline condition is formed by consecutive irradiation with an erasing power (Pe) beam which is on the intermediate level between the peak power (Pw=Pp) beam and the bias power (Pb) beam. For a space or spaces, an erasing power beam may be binarized for irradiation in a pulse pattern.
When a recording layer is irradiated with a pulse train containing a peak power beam and a bias power beam, the recording layer is repeatedly fused and quenched to form an amorphous mark. When a recording layer is irradiated with an erasing power beam, the recording layer is fused and slowly cooled or annealed in a solid phase condition to be crystallized, and then a space is formed. A pulse train containing a peak power beam and a bias power beam is typically categorized into a leading pulse, intermediate pulses, and a last pulse. The shortest mark 3T is recorded using only the leading pulse and the last pulse, and when a mark of 4T or more is recorded, the intermediate pulses are used in addition to the leading pulse and the last pulse. The intermediate pulse is called as “multi-pulse” and is set for each 1T cycle, and every time the mark length increases in length by 1T, the number of pulses is increased by one. Namely, the number of pulse trains is a value of (n−1) with respect to the mark length nT.
When information is recorded at high-speeds faster than 4× DVD, the time of a basic clock cycle T is shortened, and thus the load burdened on the light source driving member is increased. When a recording layer is irradiated with a pulse train of a 1T cycle, both the heating time and the cooling time are shortened, which leads to a problem that amorphous marks formed in sufficient size cannot be obtained. To prevent the problem, there are various proposals, in which the number of pulses for forming amorphous marks is reduced, namely, the pulse cycle is set to have a cycle longer than 1T, to secure sufficient time for both heating and cooling so that amorphous marks in sufficient size can be formed (for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2002-237051, 2002-288837, and 2001-331936, and other documents).
In addition, in the high-speed recording, information can be recorded with a low jitter at the first recording, however, when repeatedly recorded, an event that the jitter is drastically raised conspicuously occurs. FIG. 1 shows an example of variations in jitter during a random pattern being repeatedly recorded. The jitter drastically increased at the first time of repetitive recording is gradually lowered by around 10 times repetitive recording and is settled, and the variations in jitter from the time when the layer quality is degraded by the several thousand times repetitive recording to the several million times repetitive recording till the time when the jitter is raised again are small. The raise of jitter in the early stage of repetitive recording is also observed in low-speed recording as fast as 1× DVD to 2× DVD, however, the variations in jitter is not so conspicuous, and even when the jitter is raised in low-speed recording as fast as 1× DVD to 2× DVD, it is possible to satisfy the standard jitter value. As the recording linear velocity is speeded up, there are often cases where particularly the jitter at the first time of repetitive recording is high and is beyond the standard jitter, although excellent recording can be performed at the first recording or after repeatedly recorded 10 times or more.
It is presumed that such an increase of jitter in the early stage of repetitive recording is caused by some difference in conditions between the early stage crystalline phase formed in the initialization process and the crystalline phase formed during recording. For this reason, in the early stage of repetitive recording in which the early stage crystalline phase and the crystalline phase formed during recording are mixed, variations in shape of a mark occur, resulting in increases in jitter values. It is presumed that almost entire recording tracks are formed with the crystalline phase formed during recording by around ten times of repetitive recording, and thus the variations in shape of a mark are mitigated to lower the jitter.
Thus, the jitter in the early stage of repetitive recording largely differs depending on the early stage crystalline condition. The jitter in the early stage of repetitive recording can be lowered by making the early stage crystalline phase in a condition similar to the condition of the crystal formed during recording. Specifically, when initializing a recording layer for high-speed recording, the jitter in the early stage of repetitive recording tends to be lowered by using a beam of large aperture to scan the recording layer at higher linear velocity to fuse and crystallize the recording layer.
However, the crystalline condition formed in high-speed recording is typically unstable, and even with an optical recording medium that shows excellent properties immediately after the initialization process, there is a tendency that the condition changes with the lapse of time, and excellent recording will not be performed under the same recording conditions. The appearance is shown in FIG. 2. FIG. 2 shows an example that recording was enabled with low jitters immediately after the initialization even in the early stage of repetitive recording, however, when information was recorded again in non-recorded portions one-month after the first repetitive recording under the same recording conditions, the jitters was raised. Thus, it is necessary to reduce the jitter in the early stage of repetitive recording, even when the early stage crystalline condition is not equal to the condition of crystal formed at the time of recording.
As a recording method effective in reducing the jitter at the first time of repetitive recording, Japanese Patent Application Laid-Open (JP-A) No. 2004-46956 describes that the start time of a heating pulse of the leading part is delayed by 1T or more. According to studies by the inventors of the present invention, the recording method is an effective method when a cycle of 1T is used for a pulse train. However, the recording method is ineffective when the pulse cycle is set to be longer than 1T for responding to higher speed recording.
In addition, Japanese Patent (JP-B) No. 3223907 discloses that recording can be excellently performed by prolonging the irradiation time of a heating pulse of the leading part. The recording method is also effective in reducing the raise of jitter in the early stage of repetitive recording. However, in the case of high-speed recording, the recording method has a problem that the layers easily suffer from deterioration with prolonged irradiation time because the peak power value is typically high, and the repetitive recording endurance degrades. The recording method also has a problem that marks recorded in adjacent tracks are partially erased, and frequency of cross-erase is increased.
Phase-change optical recording media such as compact discs-rewritable (CD-RW) typically have a basic structure in which a recording layer containing a phase-change material is formed on a plastic substrate, and a reflective layer capable of improving light absorption of the recording layer and having thermal diffusion effect is formed on the recording layer, and the layer structure is irradiated with a laser beam from the substrate surface to thereby record and reproduce information.
Phase-change recording materials used for recording layers of phase-change optical recording media induces phase changes between a crystalline condition and an amorphous condition by repeatedly heating the phase-change material by means of a laser beam and cooling the phase-change material, and the materials are in an amorphous condition when rapidly heated and quenched, and the materials are in a crystalline condition when slowly cooled. Phase-change optical recording media are those to which the characteristic is applied to recording of information, and information is reproduced by utilizing difference in reflectances caused by difference in optical constants between a crystalline condition and an amorphous condition.
A phase-change optical recording medium typically contains an upper protective layer (which may be referred to as upper dielectric layer) formed between a substrate and a recording layer, and a lower protective layer (which may be referred to as lower dielectric layer) formed between the recording layer and a reflective layer for the purpose of preventing oxidation, transpiration, and deformation caused by application of heat by means of irradiation of a light beam. These protective layers have a function to control optical properties of recording media by controlling the thicknesses of the layers. Further, the upper protective layer also has a function to prevent the substrate from softening by heat generated when the recording layer is recorded.
Recently, with increased amount of information used through computers or the like, the recorded volume with signals recorded on rewritable optical discs such as DVD-RAM, DVD-RW, and DVD+RW are increased, and high-densification of signal information is rapidly addressed. As of now 2005, a compact disc (CD) has a storage capacity of around 650 MB, and a digital versatile disc (DVD) has a storage capacity of around 7 GB, however, demands for high-density recording are expected to be further increased in the near future.
As a method for high density recording by using such an optical recording medium, for example, there have been proposed a method in which a laser wavelength to be used is shortened to blue-laser wavelengths, and a method in which the spot size of a laser beam to be used for irradiating an optical recording medium is reduced by increasing the numerical aperture (NA) of an object lens to be used for pick-up for recording and reproducing information. Studies and developments of these methods have progressed and come close to being put into practical use.
On the other hand, as a method for improving an optical recording medium itself to increase the storage capacity, various proposals have been presented for a two-layered optical recording medium which is produced by a method in which two information layers respectively have at least a recording layer and a reflective layer disposed in a laminar structure on one surface of a substrate, and these information layers are bonded through an ultraviolet curable resin, and the like. An intermediate layer being bonded between the information layers has a function to optically separate the two information layers, and since a laser beam used for recording and reproducing needs to reach the inner side information layer as far as possible, the intermediate layer contains a material which prevents absorbing light as far as possible. However, with respect to the two-layered optical recording medium, there are still many problems to resolve. For example, when a laser beam is not sufficiently transmitted to the information layer (a first information layer) formed on the front side as viewed from the laser beam irradiation side, information cannot be recorded nor reproduced in the recording layer of the inner side information layer (a second information layer), and thus the reflective layer constituting the first information layer needs to be an extremely thin translucent reflective layer.
Recording on a phase-change optical recording medium is performed in the way where a phase-change material used for a recording layer is irradiated with a laser beam to heat the phase-change material up to the melting point or higher temperatures and then the phase-change material is quenched to change a crystalline condition into an amorphous condition to thereby form recording marks. Information is erased by heating the phase-change material of the recording layer up to the crystallization temperature or higher temperatures and then annealing the phase-change material to change an amorphous condition to a crystalline condition.
In a conventional single-layer optical recording medium, since a reflective layer can be sufficiently thickly formed, it is possible to allow residual heat caused by laser beam irradiation to quickly escape. Thus, amorphous conditions are easily formed due to progressed quenching effect. Similarly, with respect to a two-layered optical recording medium, there is no need to make a laser beam transmitted to a second information layer thereof, and thus a second reflective layer and a second recording layer may be formed to respectively have a thickness as can be seen in conventional single-layer optical recording media, and when a first information layer has a high transmittance, it is possible to obtain excellent recording properties as well as to reproduce information with ease.
However, when information is recorded on a first information layer of a two-layered optical recording medium, and an extremely thin translucent reflective layer having a thickness around 10 nm is used, it is difficult to form amorphous marks, because thermal diffusion effect is reduced. To allow information to be recorded and reproduced on the recording layer of a second information layer, it is preferred that the first information layer have a high light transmittance. Then, to record or erase amorphous marks on the first information layer of a two-layered optical recording medium, it is necessary to irradiate the first information layer with a laser beam of a recording power or an erasing power higher than those of a single-layer optical recording medium in which reflective layers can be thickly formed. For example, it is known that an erasing power Pe ranging from approx. 6 mW to 9 mW brings about excellent properties on conventional DVD single-layer optical recording media at a recording linear velocity ranging from 3.5 m/s to 27.9 m/s. Contrarily, an erasing power Pe used for irradiation of a first information layer of a DVD two-layered optical recording medium needs approx. 6 mW to 9 mW at a recording linear velocity ranging from 3.5 m/s to 14 m/s, and it needs an erasing power having high-density energy in the context of the recording linear velocity being slower than that of a single-layer optical recording medium.
In addition, since a two-layered optical recording medium needs a high-erasing power Pe but also has a thin first reflective layer, thermal diffusion property of the first information layer is fairly poorer than that of a single-layer optical recording medium, and there is a problem with thermal effects that a first recording layer being formed ultrathin suffers. When information is recorded at a recording linear velocity ranging from 3.5 m/s to 14 m/s, a recording power Pp having power two times or more of the erasing power is required, residual heat generated by such a high-recording power further causes thermal damage onto the first recording layer, and thus there is a problem to prevent the thermal damage.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2004-63005 proposes a method in which an erasing power beam is binarized for irradiation in pulse energy when a space is formed. However, in the proposal, the erasing power immediately anterior to the leading pulse having a peak power Pp for forming marks is not reduced to the bias power Pb level, and when information is recorded by the recording method, there is a problem with excessive application of heat induced by residual heat.
In addition, when information is recorded by a recording method in which the erasing power used just before forming a recording mark is temporarily increased as seen in Japanese Patent Application Laid-Open (JP-A) No. 2002-288830, it also causes a problem with excessive application of heat.
In a method that only the bias power Pb is set immediately anterior to the leading pulse, as can be seen in Japanese Patent Application Laid-Open (JP-A) Nos. 2001-273638, 2004-47053 and 2005-63586, when recording and erasing is performed in the first information layer having poor thermal diffusion property as seen in a two-layered optical recording medium, the effect is insufficient, and there is a problem that it is hard to obtain excellent recording properties, although the effect can be sufficiently exerted with a single-layer optical recording medium.
Thus, when information is recorded on a first recording layer formed in a first information layer disposed on the front side as viewed from the laser beam flux irradiation side of a multi-layered optical recording medium which contains two or more information layers on which information can be rewritably recorded and reproduced by irradiating the information layers with a laser beam to induce optical changes onto recording layer materials, a recording method for a multi-layered optical recording medium capable of preventing thermal damage that the first recording layer suffers, appropriately recording and erasing information, and having excellent recording property has not yet been provided. Therefore, prompt developments on such a recording method are desired.